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Naval glazing Rick Mather and bdp have produced a design of structural bravado in Greenwich By Keith Brownlie. Photographs by James Morris

6 May, 1999 | By Keith Brownlie

In simpler times, a visit to the Royal Observatory at Greenwich involved the prospect of London viewed axially through Inigo Jones' Queen's House and the Royal Naval Hospital to Father Thames beyond. These days there is much to divert the eye from the straight and narrow, with structures labelled xxl appearing across the eastern horizon from the peninsula to the wharf. Back on axis, however, another new outsize construction looms large, rising from within the extant fabric of the National Maritime Museum like a cake mix rising in a tin.

This architectural cuckoo's egg is the giant glazed enclosure of Neptune Court, a development harvesting new habitable space from the unexploited void of the building's original plan. The scheme is by Rick Mather Architects (brought in by the Heritage Lottery Fund) with bdp in a masterplanning and multi-disciplinary role. It is a rationalisation and expansion of the existing building to create a new museum, aimed at clarity and cohesion internally, and outwardly providing a distinctive identity among the architectural riches of Romney Road. This is achieved, in the main, by the organisation of galleries and amenities around a new multi-purpose space, internalised by the sizeable clear-span roof which so defines this scheme.

The museum, reportedly 'crippled' by orientation and circulation problems in its previous guise, has undergone substantial architectural surgery to make sense of the space. Intricate stitching of disparate levels in existing galleries and the subtle manipulation of listed fabric are unsung relative to the decision to treat the building to a new hat. Clearly the architects have worked hard in the buildings surrounding Neptune Court to achieve the effect of effortless intervention. The insertion of new openings into listed walls, the fusion of new and existing fabric and the ordering of disorderly spaces are executed successfully enough to go almost unappreciated. It is Neptune Court, then, that gives architectural identity to the new National Maritime Museum, defining it visually, spatially and organisationally.

Visitors are drawn from the museum's opposing entrances immediately and unavoidably into the building's heart, either at ground level or to the mezzanine which resolves the level change across the site and inhabits the majority of the court's interior. The naturally ventilated space is a light-soaked cavern, its ambience defined by the unifying planes of the roof above and the field of pietra serena that carpets the floor at both levels. The architecture is stripped and rigorous, from the white cubism of the new northern block to the restrained detailing of elements which lends the project a rather serious demeanour. The perimeter moat left by the mezzanine at ground-floor level gives on to exhibition 'streets' on three sides, while the court itself plays home to a smattering of large exhibits at both levels. These few items, accompanied by the mezzanine al-most-fresco cafe, appear lonely within the voluminous enclosure. This modest use of available space is ironic given the single-span roof which clears the parapets of the existing facades. The column-free arrangement, a solution pursued doggedly by the architect, cannot be justified by the need to increase useable space, given the spartan spatial reality. The roof spans clear across Neptune Court, ultimately, because it was achievable. To have realised a lesser solution would be, contextually speaking, to have missed the boat.

The gallery wings, which face on to the courtyard, line the space with refurbished facades whose stuccoed solidity provide the canvas for a web of shadows cast by the roof. In contrast, the fourth side, to the north, comprises a new construction of polite modernism which houses a new entrance at ground level. To the outside, this block of accommodation retains an existing facade centrally, gripped by fully-glazed flanks which reveal something of the internal volume to the approaching visitor. This is a necessary device given the planners' predictable assertion that the new roof should not manifest itself above the parapets of the existing. This constraint does much to inform the geometric signature of the roof, with its shallow arched pillow profile, but is pointless conservatism in the light of the structure's highly visible aspect to Greenwich Park.

The project's grid-shell roof is a structure of interlocking spans, a typology with its history in the geometry of geodesic domes and latterly more complex free forms such as Frei Otto's seminal timber grid roof at the Mannheim Bundesgartenschau. Use of this form of structure has visibly gathered pace, with examples such as Jorg Schlaich's delicious courtyard covering at the Hamburg Museum of History setting the stall for future developments. Both bdp and Rick Mather Architects have had prior relevant experience, the former in the new Wimbledon No 1 Court's doubly curved circular grid shell roof, and the latter, schematically, in its second- placed design for the British Museum Great Court. Neptune Court itself is of such a scale that there are few current comparables, but the eventual completion of the Great Court development will at least make for an interesting architectural dialogue.

At Neptune Court the roof comprises a single-curved arched profile, exploiting the necessary economy of repetitious geometry. The resultant shape therefore inevitably lacks the fluidity of a double-curved surface and relies upon the resolution of two dimensionally curved planes. In short, it has seams. The geometric consequence of springing arches from each of the rectangular court's four edges is essentially a barrel vault with hipped ends, an ignominious description given the relative subtlety of the realised form. From above, there is something likeable in the way that the geometry of the slick glass skin resolves itself from the orthogonal to the organic. The roof rises from each corner with creases as sharp as trouser legs fresh from the press, eventually resolving to the shallow arch form which lends the impression of an inflated structure.

Internally, where the structural grid of the roof rules the aesthetic, the particular and substantial problems of spanning the 45m x 54m court dictate the configuration of the steelwork. Consider the common analogy for a grid shell in its unadulterated form as a wire kitchen sieve. The Neptune Court roof broadly takes this form of members interlocking to form the foundations for a tessellated skin of glass squares. The demands of the span then dictate the addition of certain structural members to the mix. The horizontal thrust generated by such shallow arches is resolved by a matrix of tie rods beneath the roof in the horizontal plane. Additionally, whereas double-curved structures like the No 1 Court inherently resist buckling, here alternate arched members take the form of trusses to account for these out-of-plane forces. The net result, of course, is that the clarity of the system is diluted as the roofscape busies itself with the job in hand. In fact, the roof is of such a scale that this does not have too severe an impact on the overall effect and, while the structure lacks the lightness and legibility of the kitchen sieve, the structure is a considerable act of architectural and structural bravado.

But the overall impression is that the National Maritime Museum has been delivered a product of restraint, and one could wish for the design to be less well-mannered at times, to have more sea legs, as it were. Refer to the guidebooks, however, and you'll note that the sixteenth-century Queen's House, home to the museum from 1937, 'is remarkable for its architectural restraint and [lack of] contemporary Elizabethan elements'. Inigo Jones would surely approve.

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